The term "anaesthetic system" or anaesthetic breathing system, when used in this specification, is taken to mean the connecting apparatus between the anaesthetic machine and the face mask or endotracheal tube which connects to a patient, except where the context indicates otherwise.
Anaesthetic breathing systems are classified into 3 main groups, namely those which employ a circle system in which anaesthetic gases are reused by removing carbon dioxide due to circulation through a container containing soda lime (or similar substance); those which do not recycle gases and vent all expired gases to the atmosphere and do not permit any rebreathing by the use of two unidirectional valves in the inspiratory and expiratory side of the system; and those which do not recycle gases but vent expired gases to the atmosphere but nevertheless permit a degree of rebreathing. Of the last group all requirements for the administration of anaesthesia can be met with a number of different breathing systems, each having its own special application. Mapleson generally classified these systems as A, B, C, D and E and various analyses by research anaesthetists have revealed Mapleson A systems to be generally efficient for spontaneous respiration in adults, D systems to be generally efficient for controlled ventilation in adults and children and E systems to be advantageous though not necessarily efficient in children for both spontaneous and controlled ventilation. Systems B and C offered no special advantage at all. Hence to cover all eventualities encountered by the anaesthetist, Mapleson A, D and E systems are employed. Examples of Mapleson systems are the well known Magill (Mapleson A), the Bain (Mapleson D/E) and Ayre's T-piece (Mapleson E).
The Mapleson A or Magill system, is used for spontaneous respiration on adults and comprises a flexible tube delivering fresh gas from the machine, a reservoir bag at the machine end, and a pop-off valve, which is a simple relief valve, adjacent the patient's face. For controlled ventilation on adults, the Mapleson D/E or Bain system is used. This system comprises a fresh gas supply tube located within the bore of an expired gas removal tube at the distal end of which is located a relief valve and reservoir bag. Expired gases may be collected at the relief valve and removed from the theatre. The reservoir bag can be removed to allow connection to an automatic ventilator from which the expired gases may be removed from the theatre. In pediatrics, a simple T-piece (Mapleson E) is generally used for spontaneous respiration and controlled ventilation.
It is clear that each particular system is of limited application. In addition, the Magill and the T-piece systems contribute to the atmospheric pollution of the theatre by the introduction of expired respiration products and anaesthetic gas into the theatre at a point close to the patient from where it is cumbersome and inconvenient to attach a scavenging device. Only the Bain system allows easy scavenging back at the anaesthetic machine and removal of these gases out of the theatre with consequent reduction of this hazard.
A later system, based on the Mapleson A principle and developed by Lack, addressed the problem of theatre pollution by the provision of a take-off tube for expired gases coaxially down the fresh gas supply tube. The take-off tube exits from the bore of the supply tube at a location remote from the patient where a relief valve is arranged to blow off when the expiration pressure of the patient overcomes the resistance of the valve mechanism. The Lack system can only be used in the same situations as the Magill.
Recent attempts have been made to design an anaesthetic system which will have universal efficient use. Combination of a circle system used for spontaneous respiration and a Mapleson D system for controlled ventilation still limit its use to adults and requires soda lime with all its consequent disadvantages familiar to those skilled in the art. Another idea has been to use a closed loop without soda lime in which the separation of dead space gas (gas which has entered the lung but never reaches the respiratory gas exchange areas; it therefore has the same composition as fresh anaesthetic gas) from alveolar gas (which contains carbon dioxide exchanged from blood in the deeper parts of the respiratory passages) is achieved by a one-way valve on the expiratory limb immediately adjacent to the patient's face with two expiratory tubes from this valve to the anaesthetic machine. One tube carries dead space gas back to the beginning of the inspiratory limb thus completing the loop, while the other expiratory tube carries the alveolar gases to a pop-off valve from which these gases are vented from the system. A back-pressure mechanism on this valve permits controlled ventilation with the ventilator placed with the inspiratory limb of the loop. This device is not suitable for children because of its bulk and high airway resistance due to its complexity with the presence of a valve. Yet another approach to preserve dead space gas has been the introduction of a jet fluid control device at the patient end of a breathing system. This narrow jet directs the first gas to be expired (dead space gas) at a high velocity back down the inspiratory limb. As the pressure of gases rises in this limb, so the remainder of the expired gas (alveolar) is forced into the expiratory limb which is open to atmosphere at the anaesthetic machine. This, however, proved to function less efficiently than conventional systems, for the jet created such high turbulance within the system that it effectively mixed both dead and alveolar gas with the total loss of the former which now contained carbon dioxide. In conclusion not one multi-purpose anaesthetic breathing system exists that is efficient and can be used for all patients irrespective of age.
Applicant is aware of the following U.S. Pat. Nos. 3,901,230 (Henkin) which relates to a closed loop system without the use of soda lime already described above; 4,281,652 (Miller) which relates to a jet fluid control device at the patient end of a breathing system also referred to above; 3,993,059 (Sjostrand) which relates to a high frequency positive ventilation system; 3,814,092 (Simionescu et al) which relates to a portable anaesthetic machine; 4,007,736 (Schreiber) which describes a ventilator with a timing mechanism and 4,249,527 (Ko et al) which relates to a pressure administrating apparatus which maintains a positive pressure in the lungs of a patient, and United Kingdom Patents Nos. 734,639 (Svenska Aktiebolaget Grasaccumulator) which relates to a closed and open circuit anaesthetic system and 1,272,583 (Blease Medical Equipment) which describes a ventilating machine designed to facilitate sterilization after use.
It is an object of this invention to provide a system which will find application in each and all of the circumstances mentioned above and which will, moreover, be as efficient, or considerably more efficient, than the systems presently in use in some or all of the applications referred to.
It is further the object of this invention to facilitate the scavenging of waste expired anaesthetic gases in all the applications mentioned above, thus reducing theatre pollution and its consequent danger to theatre staff to minimal levels or even eliminating pollution completely.
Yet another object of this invention is to provide an anaesthetic breathing system suitable for anaesthesia administration in both the sophisticated academic environment and remote rural areas. By virtue of the properties of the apparatus the anaesthetist can use very economical anaesthetic gas flows even without carbon dioxide absorption and yet he does not prejudice the safety of his patient. He can predict the carbon dioxide tension of this patient during controlled ventilation even when the latter is performed by hand. The invention is robust, simple and easily sterilized and consequently has many advantages over the circle absorber system which, unless specially adapted, is also unsuitable for children.
Yet another object of this invention is to provide an apparatus which is cheaper, easier to use, and less bulky than a combination of currently used anaesthetic breathing systems required for anaesthesia in the four circumstances mentioned earlier. Further this invention eliminates the use of soda lime and thus also the cost, transportation and the difficulties often encountered in remote areas in obtaining it.